This invention is generally directed to improvements in television receivers. It is specifically directed to an improved noise processing system for use in such a receiver.
Television receivers typically include a video detector for developing a composite video signal which includes video information and sync information. The former information is coupled to a video processing path which amplifies and otherwise pre-conditions the video information for input to a cathode ray tube. The sync information is extracted from the composite video signal by coupling the detector's output to a sync processing path which normally includes a sync separator. The latter device develops horizontal and vertical sync output pulses from the composite video signal.
In the case where impulse noise is present in the detector's output, that noise may be coupled into the video processing path and result in a noisy image being developed on the screen of the cathode ray tube. The impulse noise will also be coupled into the sync processing path and cause the sync separator to generate unwanted outputs. Because the output of the sync separator is usually coupled to an AGC (automatic gain control) system in the receiver, the latter system may be disrupted by the noise-induced output of the sync separator.
To avoid AGC disruption by impulse noise, many receivers include some form of noise protection, as shown in U.S. Pat. No. 3,806,646, for example. Although prior noise protection circuits have provided improved AGC performance under noisy conditions, even better protection is desired, particularly for receivers which use envelope detectors. Such detectors tend to rectify white-going noise impulses and develop higher energy black-going noise impulses. The latter impulses have a particularly deleterious effect on the receiver's AGC system and sync separator, causing the displayed television image to suffer from broad area brightness fluctuations, small area brightness and color distortions, and horizontal and vertical jitter.
Accordingly, it is a general object of the invention to provide an improved noise processing system for a television receiver.
It is a more specific object of the invention to provide a noise processing system which protects the receiver's video processing path as well as its sync processing path from impulse noise.
It is another object of the invention to provide such a noise processing system which gives a high level of noise protection to the sync and video processing paths in the presence of high energy, black-going noise impulses of the type output by envelope detectors.